Metal coatings for decoration and ornamentation have been used for thousands of years. More recently, metallic pigments have become commercially important as surface coatings. Metal-coated surfaces can produce aesthetic, bright, metallic finishes that can also withstand environmental conditions and weathering better than many other surfaces or surface coatings. The high cost of metals, such as gold and silver, encourages the use of thin leaf products for surface coatings. Thin leaf includes a sheet less than about 2 mm thick and typically less than about 0.1 mm thick. For example, gold leaf can be less than about 0.001 mm thick. Producing such thin leaf can be difficult and can include hammering the metal to a suitable thickness. The labor required for this process and the malleability of the metal limit the practicability of such thin leaf products. Historically, the expense of thin leaf metallic coatings had been limited its use to jewelry, porcelain, chinaware and other art objects.
An alternative to metallic leaf includes a mixture comprising thin metal flakes and a drying oil. After applying the mixture to a surface and drying, the mixture can resemble a continuous metal coating. The metal flake can be produced by any convenient process, including grinding, stamping, rolling, and milling. Reactive metals, such as aluminum, can be produced in an anaerobic environment such as in organic oil. One process for producing metallic coatings includes forming a suspension of metal particles in a suitable liquid and applying the suspension using conventional techniques such as painting or printing. This process is convenient but the resultant coating can lack the reflectivity and aesthetics of a surface coated with a true metal leaf. The shape of the metallic particles and their distribution in the dried suspension are believed to affect reflectivity.
A thin, reflective metal flake can be formed using vapor deposition, such as vacuum vapor deposition. Flakes produced by this process can better mimic traditional metal leaf. Vapor deposition can include heating a metal in a vacuum to form a metal vapor and exposing a surface to the vapor. The vapor condenses on the surface to form a metal film. The surface may be chilled to facilitate condensation. Varying factors such as the vapor pressure, temperature gradient between the vapor and the surface, and the residence time of the surface in the vapor control the thickness of the metal film and the resultant metal flake. The thickness of the metal film is usually less than 10 microns, and more usually less than 1 micron. Vapor deposition processes include methods such as thermal evaporation, electron beam evaporation, condensation, sputtering, or combinations thereof
After deposition, the metal film can then be removed from the surface. Commonly, the surface includes a release coating and the metal film condenses on the release coating. The surface is passed through a solvent system which dissolves the release coating and releases the metal film into the solvent. A suspension of metal flake in the solvent is formed from which the metal flake can be separated. The metal flakes can be used in coatings, such as paints and inks, or to impart optical, mechanical or electrical properties to a product either as a coating on the product or incorporated into the product.
A common method of producing metal flakes deposits the metal film onto a surface consisting of a moving web. The web can be spooled between two reels and the process can be carried out continuously over the web. Prior to metallization the web can be coated with a dissolvable coating to better facilitate the separation of the vacuum deposited metal layer from the web. The web is thereafter soaked in a solvent solution wherein the metal film is separated from the web to produce metal flakes. This method of producing metal flakes, however, has not been wholly satisfactory. First, the method is discontinuous and can be slow because the movement of the web must either be periodically halted or the movement slow enough so that the web can be soaked in the solvent solution for some appreciable time. The nature of the process decreases production. Second, the necessity of using a web represents a considerable capital and maintenance cost because the web must frequently be replaced.
In an embodiment of this method the base substrate consists essentially of a web. The web is coated with a soluble release coating, metallized in a vapor metallization process, and repeatedly coated with release coating and metallization to create a stack of release coating and metal layer on a single web. The web is typically disposed of after the removal of the metal flake, so multiple layers on a single web can reduce amount of web needed and, consequently, the cost. Although this embodiment reduces the cost for the base substrate, there is still considerable expense as the structure must be moved repeatedly between a coating system for the application of the soluble coating and the vacuum metallization system. This increases handling and time to produce a metalized layer.
Prior art has attempted to remove the use of a base substrate to produce free standing thin film particles. This process includes coating the surface of a deposition drum with a solid release agent, such as for example a wax, and depositing a metal film onto the wax. A knife or doctor blade can scrape the release agent and the metal film from surface in a continuous process. In a typical embodiment, the moving surface is a rotatable metallic drum disposed within a vacuum chamber. Located about the periphery of the drum is at least one vacuum deposition station. The station distributes the metal film onto the surface of the rotating drum. After the metal film and release agent is scraped from the surface, the residual release agent is removed from the metal film typically in a solvent washing process. The metal film, which fractures into thin metal flake, can be separated from the solvent by, for example, evaporation or centrifugation. The solid release agent can be recycled to the vacuum chamber for redeposition onto the rotatable drum.
Alternatively, prior art has vapor deposited a release agent onto a coating drum and subsequently deposited a metal layer. The process can be a continuous process and can create a plurality of layers of release agent and metal. The multilayer stack can be removed from the deposition drum and subsequently crushed to better expose the release layer to the solvent process.
While such embodiments can overcome certain limitations of the vapor deposition process, in particular the omission of the use of a disposable and costly moving web, problems remain. For example, the processes can require vapor deposition of the release layer so that special evaporation and deposition equipment is required. The release agents described within the known state of the art are either of solid state upon deposition onto the coating drum or are solidified prior to metallization using either electron beam or plasma exposure. A solid release agent requires a potent solvent to separate from the thin metal film. A liquid release agent would be more easily separable from the metal film, but attempts to deposit metal film on a liquid surface have been ineffective.